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Sunday, 3 August 2014

Googong Dam and London Bridge

The Queanbeyan River has been dammed to form a large reservoir called Googong Dam. Further upstream of the Dam, a tributary of the Queanbeyan River has an arch called London Bridge.

Googong Dam

Sitting upstream of the Australian cities of Canberra and Queanbeyan, on the Queanbeyan River, is the Googong Dam. The Australian National Committee on Large Dams has placed the dam in its hazard category of ‘extreme’ - because of its location. If the dam wall were to collapse, significant damage would occur to both cities.

Originally the Federal Capital, now called Canberra, was to be situated in a territory that fully encompassed the local water catchment. At the eleventh hour, New South Wales reneged on the proposal, and cut the land to be granted to the Territory in half, leaving the water catchment bifurcated between Canberra to the West and Queanbeyan to the East. However, New South Wales was forced to surrender all water rights in the East to the use of the Federal Capital - a bizarre solution, the ramifications of which are yet to be fully explored.

One result saw the construction of a huge reservoir, the Googong Dam. This was to be used as water storage for Canberra although it was outside the boundaries of the Federal Capital. While located in New South Wales, the dam was to be managed by those responsible for the administration of the capital.

By 2007, at the height of the drought in the region, the dam levels had fallen significantly. At that time, in part because of other works, work was planned for Googong Dam's wall. Earlier floods had significantly eroded the spillway, possibly impacting on the structural integrity of the dam wall. When the repair work commenced, extensive cracking was exposed in the upstream retaining wall, indicative of structural failure of the wall section. The detailed report on the structural challenges faced by those charged with the repair is worth reading - and has some good shots of the spillway before and after remediation.

Suddenly, in late 2010, climatic conditions reversed. The long drought was broken. Rain filled the dam and, by Friday, 3 December 2010, a storm event saw a massive 700 cubic meters of water per second flow over the top of the repaired wall and spillway. While the region was inundated, and central Queanbeyan was closed as water coursed through the city, the dam wall held. A far greater crisis was averted.

London Bridge

A limestone karst (a landform formed by the dissolution of limestone) hollowed by the Burra River. A short walk after a short drive from Canberra - but seldom visited.